1. Field of the Invention
This invention relates to electron beam projection (E-beam) systems and more particularly to an E-beam system which exposes targets to write both large and small patterns on a target, i.e. work piece.
2. Related Art
E-beam exposure systems have been employed for micro-fabrication of large scale integrated circuits on semiconductor substrates. Such systems are useful for writing patterns on radiation sensitive material usually composed of photoresist, deposited on targets such as substrates in the form of semiconductor wafers or photolithographic masks. The E-beam exposes the radiation sensitive material and a pattern is developed on the wafer or in the mask.
The E-beam system must balance the need for maximum throughput of work pieces with the ever increasing demands of industry for smaller and smaller microscopic patterns. The typical E-beam system of this kind includes an E-beam source, a deflection system for deflecting the E-beam in a predetermined pattern and magnetic projection lenses for focussing the E-beam.
After the E-beam is deflected and focussed the beam reaches the target, i.e. work piece. In the past, the target has been subdivided into larger areas and smaller areas within which different E-beam deflection stages deflect the E-beam within larger areas on the target known as fields and smaller areas on the target known as sub-fields.
An example of an early version of such an arrangement of fields and sub-fields is found in commonly assigned U.S Pat. No. 4,494,004 of Mauer et al, entitled "Electron Beam System", which describes a method of using a shaped-beam E-beam system with magnetic deflection yokes for the sequential, rectilinear scanning of sub-fields, and electric deflection plates for vector scanning within each sub-field. Square apertures in plates provide a shaped spot.
Another aspect of this type of system is that it is desirable to eliminate E-beam projection system aberration. In U.S. Pat. No. 4,376,249 of Pfeiffer et al entitled "Variable Axis Electron Beam Projection System", a variable axis E-beam projection system is described in which the electron optical axis is shifted so as to be coincident with the deflected E-beam at all times. Shifting the E-beam optical axis has the advantages that it (1) causes the E-beam to land perpendicular to the target and (2) eliminates lens aberration caused by off-axis E-beams. In particular a projection lens is arranged so that upon pre-deflection of the E-beam by deflection yokes, the electron optical axis of the lens shifts to be coincident with the deflected beam.
Pfeiffer et al 4,376,249 also describes a system in which the E-beam is deflected and a magnetic projection lens, which has a rotational symmetry, focuses the deflected beam. A pair of correction yokes positioned within the bore of the projection lens produce a magnetic compensation field proportional to the first derivative of the axial magnetic field strength distribution lens to shift the electron optical axis of the projection lens so that the axis of the E-beam remains coincident with the shifted electron optical axis and so the E-beam lands perpendicular to the target.
U.S. Pat. No. 4,544,846 of Langner et al, commonly assigned, entitled "Variable Axis Immersion Lens Electron Projection System", known as "vail", is an improvement on U.S. Pat. No. 4,376,249 above. It also shifts the E-beam as does the '249 patent, while eliminating rapidly changing fields, eddy currents, and stray magnetic fields in the target area. In the case of the Langner et al system, the vail lens includes an upper pole piece and a lower pole piece each of which includes a non-zero bore section, a zero bore section, and an opening between the zero bore section and the lower pole piece for inserting the target into the lens. The magnetic compensation yoke is positioned within the bore of the upper pole piece to produce a magnetic compensation field which is proportional to the first derivative of the axial magnetic projection field.
Co-pending U.S. patent application Ser. No. 142,035 of Groves et al for "Telecentric Sub-Field Deflection with Vail" now U.S. Pat. No. 4,859,856, describes a vail system similar to that of Langner et al, supra, wherein there are upper and lower deflection stages with the upper stage comprising electrostatic deflection plates for deflecting a pattern within a sub-field. The lower deflection stage is comprised of magnetic yokes which deflect the beam within a field. The electrostatic deflection plates are located well above the back focal plane of the vail lens in order to accommodate the vail system (or its equivalent.)
In Groves et al, placing the electrostatic plates in the vail lens is not possible, since the space available near the back focal plane of the vail lens is extremely limited.
A paper by Saitou et al "Electron Optical Column for High Speed Nanometric Lithography" to Hitachi, describes a three stage deflection system which includes a third electrostatic deflector which employs a round gaussian spot which must be scanned in small raster to expose a rectangular area, which is accomplished in a shaped beam system with a single exposure. FIG. 2 of Saitou et al shows the three stages with the three writing methods including "Variable Gaussian 3-Stage", "Variable Shaped 2-Stage" and "Fixed Gaussian 1-Stage". It shows a wafer with chips broken up into fields and sub-fields.
An article by Thompson, Liu, Collier, Carroll, Doherty and Murray in "The EBES4 Electron-Beam Column" of AT&T Bell Laboratories describe a triple deflection system with a magnetic deflection telecentric first stage followed by two electrostatic stages.
Alles et al, "EBES4 A New Electron-Beam Exposure System" J. Vac. Sci Technol. B5(1) January/February 1987 states that the variably shaped beam is not used in the EBES4 system, but that small fixed spots are used as the building blocks.
U.S. Pat. No. 4,390,789 of Smith et al for "Electron Beam Array Lithography System Employing Multiple Parallel Array Optics Channels and Method of Operation" describes a two stage deflection system with both fine and coarse deflection stages. The system uses a fly's eye system with lenslets. It includes two channels and no shaped beam. A matrix of discrete lenses is employed and the axis is not shifted.
U.S. Pat. No. 4,514,638 of Lischke et al entitled "Electron-Optical System with Variable-Shaped Beam for Generating and Measuring Microstructures" has three electrical deflection systems AE1, AE2 and AE3, the second and third of which return the E-beam to its optical axis. The second and third systems are not related to positioning the beam on the target.
U.S. Patent No. 4,465,934 of Westerberg et al for "Parallel Charged Particle Beam Exposure System" shows use of a double octupole deflection system in FIG. 2 of that patent.